i think u answered my question. i recentely replaced the galv bolts with stainless where the rails meet the floor. maybe thats reacting/ t
o oxidize and create this.
Hmmmm. Interesting. Stainless steel actually does "ok" corrosively when in contact with aluminum in a marine environment, even though they are pretty far apart on the nobility scale.( http://www.corrosionist.com/Galvanic_Series_of_Metal_in_Seawater.htm) The farther apart from each other on the scale, the more likely the two metals are to act corrosively together. What makes your case interesting is that galavanized steel and aluminum are closer together and so shouldnt, theoretically, be worse than stainless.
So let me ask these questions: 1. Where are you located? Acid rain/industrial pollutants often cause galvanized fasteners to corrode badly. Is there other evidence of white stuff on other galvanized parts in or near your boat?
2. Why did you swap the SS for galvanized?
3. Maybe its just poorly made galvanized fasteners and the zinc coating is really some other substance.
Bottom line: If ss screws didn't have a problem in the past, I would revert to those. In fact, call the factory and ask which type of stainless ( 304, 316, 318, etc they used originally) All the rest of this is just, well, all the rest.
Sorry to double post, but here's another thought. You say you turn your batteries off, but you could still have a short somewhere or current leaking. What devices are wired directly to the battery? It should only be the bilge pump (s) and engine.
That said, are there lights on the rails? Do you have an electrically-operated Bimini top? I am just trying to think of "typical" electrical devices on a pontoon. Any wires running down the rails or clipped to them.
For any of the above, get yourself a multimeter and check for current when the battery switch is off--and also on. If current is running into the rails, even only when you have the switch on, you could still have corrosion.